It is well known in the art to treat hydrocarbon wells, i.e., oil wells and/or natural gas wells, with acid solution. The acid so provided reacts with basic components, for example, magnesium and calcium carbonates, of the formation through which the well is being or has been drilled. Such reaction serves to enlarge the openings and channels through which the hydrocarbon passes during recovery and production of the hydrocarbon. As the provided acid reacts with the basic components of the formation and the pH increases, i.e., the acid "spends", various precipitates tend for form which lead to undesirable plugging of the formation.
In many cases the acid solution, after partial reaction, will contain ferrous and ferric ions and at a sufficiently high pH one or more iron hydroxides will begin to precipitate. This undesirable precipitation leads to formation plugging and has other detrimental effects, particularly when the iron hydroxide is ferric hydroxide. To control this ferric hydroxide precipitation it is known to employ an iron sequestering agent and/or an iron reducing agent to maintain the iron ions in solution and/or reduce the ferric ions to ferrous ions. Typical iron sequestering agents include citric acid and ethylenediaminetetraacetic acid.
When the hydrocarbon well undergoing acid treatment is "sour", i.e. the well product contains hydrogen sulfide or other sulfur-containing compounds, additional problems result. The presence of sulfide ions tends to reduce ferric ions to ferrous ions, a somewhat desirable result. The ferrous sulfide tends to stay in solution if the pH is sufficiently low, e.g., the solution pH is below about 1.9, but as the acid spends the pH will rise and ferrous sulfide as well as ferric hydroxide precipitates. Moveover, elemental sulfur is also formed by the ferric ion reduction and will also precipitate. The combined precipitate tends to plug the formation. As a consequence, the acidization of sour hydrocarbon wells is particularly complex. While various proposals have been made to control the undesirable precipitation which takes place during sour well acidization, the proposals have not produced entirely satisfactory results.
One such proposal designed to prevent the undue precipitation of ferric hydroxide and ferrous sulfide was to maintain the pH of the acidizing solution below 1.9 at all times. However, in most acid treatments this pH control is not easily achieved. A method and a composition for precipitate control during sour gas acidization is disclosed by U.S. Pat. No. 4,633,949. This proposal included the use of an iron sequestering agent such as ethylenediaminetetraacetic acid in combination with an iron reducing agent such as erythorbic acid as a composition to be added to the acidizing solution. The combination did not, however, prevent the formation of precipitates containing sulfur species such as elemental sulfur.
Somewhat better results are disclosed by Dill et al in U.S. Pat. No. 4,888,121 and the related U.S. Pat. No. 4,949,790. These patents, based on the same disclosures, use an iron sequestering agent and a sulfide modifier to be used in conjunction with the acidizing solution. The iron sequestering agent was a conventional agent such as ethylenediaminetetraacetic acid and the sulfide modifier is an aldehyde or related materials such as acetals, hemiacetals or other materials likely to generate aldehyde under the acid conditions used in the well treatment process. Certain of the combinations disclosed by Dill et al are comparatively effective in preventing the undesirable precipitation which normally accompanies sour well acidization. However, it would be of advantage to provide an alternate method for control of precipitation of iron and sulfur species as well as the compositions employed in that method.